1. Field of the Invention
This invention relates to a passenger protecting apparatus for an automobile such as an air bag, a seat belt pre-tensioner and the like, and further relates to an actuating apparatus therefor.
2. Description of the Prior Art
FIG. 11 is a circuit diagram showing a conventional igniting circuit apparatus of an air bag which is disclosed in Japanese Published Unexamined Patent Application No. 278558 / 93 (Tokkai-Hei 5-278558). In FIG. 11, reference numeral 101 denotes an igniting device of an air bag called a squib and composed of a resistor such as a heating coil or the like. Reference numeral 102 denotes a power supplying section; reference numeral 102a denotes a battery; reference numeral 102b denotes a power circuit for supplying a current from the battery 102a to the squib 101. Reference numeral 103 denotes a collision judging part; reference numeral 103a denotes an acceleration sensor for outputting the acceleration of a vehicle in the direction of deceleration of the vehicle as an electric signal; reference numeral 103b denotes a CPU receiving the output of the acceleration sensor 103a to judge whether the air bag should be inflated. Reference numeral 104 denotes a semiconductor switch for ignition to be controlled by the CPU 103b. Reference numeral 150 denotes a mechanical acceleration switch which can be actuated by a shock comparatively smaller than one judged by the CPU 103b as a shock to expand the air bag. Reference numeral 106 denotes a capacitor which operates as an auxiliary power supply for ignition.
Next the operation of the conventional actuating apparatus for a passenger protecting apparatus will be described. When a collision of a vehicle has occurred, the acceleration switch 150 provided for preventing a malfunction is closed at first. When the semiconductor switch 104 is closed by the collision judging part 103 after the closure of the acceleration switch 150, an igniting current is supplied from the power supplying section 102 to the squib 101 for actuating the passenger protecting apparatus or the air bag.
In the aforementioned construction of the actuating apparatus, the acceleration switch 150 is provided for preventing a malfunction in the case where the acceleration sensor 103a and the CPU 103b are faulty.
FIG. 12 is a perspective view of a conventional acceleration switch for a passenger protecting apparatus disclosed in U.S. Pat. No. 5,306,883 (Manandhar et al.). In the figure, reference numeral 151 denotes a cover. Reference numeral 152 denotes a base. Reference numeral 153 denotes a cylindrical mass (weight). Reference numerals 154 and 155 denote electrical terminals; reference numerals 154a and 155a denote pins extending from the terminals 154 and 155 respectively. Reference numeral 156 denotes a contact portion made of a blade spring. Reference numeral 157 denotes a coil spring. Reference numeral 158 denotes pins for fixing.
Next, the operation thereof will be described. In the acceleration switch 150 shown in FIG. 12, the terminals 154 and 155 are not electrically connected to each other at an ordinary time because the mass 153 is pressed by the coil spring 157 to push the contact portion 156 open. But when a force caused by an acceleration at a time of collision is impressed in the direction of an arrow A in FIG. 12, the mass 153 moves to the direction of the arrow A, and the contact portion 156 is pressed to the direction of the arrow A by the force of the blade spring to connect the terminals 154 and 155 to each other electrically. After the terminals 154 and 155 are connected to each other, the electrical connection of the terminals 154 and 155 is kept so long as a force stronger than the reaction force of the coil spring 157 is applied in the direction of the arrow A, because the mass 153 tends to move to the direction of the arrow A.
FIG. 13(a)-FIG. 13(c) are waveform diagrams showing states at a time of a collision; FIG. 13(a) is a waveform diagram showing the changes of a collision acceleration generated to the direction of the arrow A, namely the direction of the deceleration of a vehicle at a time of a collision; FIG. 13(b) is a waveform diagram showing the amount of the movement of the mass 153; FIG. 13(c) is a waveform diagram showing the state of opening and closing of the acceleration switch 150. Reference letter Gb shown as a broken line in FIG. 13(a) indicates an acceleration (bias acceleration) which generates a force having the same strength as the spring force of the coil spring 157. When a collision acceleration exceeds the bias acceleration Gb, the mass 153 begins to move as shown in FIG. 13(b). And when the amount of the movement exceeds a certain fixed quantity (shown as a broken line LON in FIG. 13(b)), the contact portion 156 electrically connects the terminals 154 and 155 to each other to turn on the acceleration switch 150. Then, the acceleration switch 150 is kept to be turned on until the amount of the movement of the mass 153 becomes smaller than the fixed quantity LON as shown in FIG. 13(c). Since the acceleration switch 150 is kept to be turned on in such a manner, the operation of the squib is prevented from being unstable or faulty because the acceleration switch alternates between on and off at a collision time.
But, since the aforementioned conventional acceleration switch 150 for the use in an actuating apparatus for a passenger protecting apparatus determines its characteristics only by means of the spring constant of the mechanical spring 157 and the weight and the moving range of the mass 153, it has a problem that it cannot change its characteristics by a large margin.
Besides, the time for keeping the electrical conduction is required to be longer than a certain degree, and the conventional acceleration switch 150 answers the request by setting the distance of the movement of the mass of the switch 150 to be long. Consequently, the length of the switch in the direction of the arrow A in FIG. 12 cannot be shorter than a certain fixed length. Accordingly, the size of the whole apparatus needs to be large, and then the switch 150 has a high cost.
Besides, in an acceleration switch opening and closing its contacts mechanically by means of a mass, there inevitably happens a chattering phenomenon in its contact portions, namely its contacts part from each other due to the reaction of contacting to be contacted and separated intermittently. That is to say, the conventional acceleration switch has a problem that the conduction thereof becomes intermittent.
Besides, if the aforementioned actuating apparatus is used in a passenger protecting apparatus having a plurality of squibs, and the power supplied to each squib is independent in order that the other squibs will not malfunction if one squib is faulted, for example by mis-ignition caused by a failure such as a short circuit or the like. For that purpose, a plurality of acceleration switches shown in FIG. 12 are required, and consequently, the space for mounting such switches becomes large, and the cost of the apparatus increases.